Doosan Infracore Corporation has developed the combustion system of a brand-new DX12 heavy-duty diesel engine equipped for 38 ~ 50 tonnage excavators and 4.2 ~ 4.5 m3 bucket-size wheel loaders which are mainly targeted to emerging countries. A variety of advanced combustion technologies were incorporated in the design of the DX12 engine to meet Tier3 emission legislation even with a mechanically controlled fuel injection equipment while ensuring around 2 % improvement in fuel consumption as well as over 8 % increase in rated power than its Tier2 predecessor. Mechanical-type diesel engine has a couple of significant advantages especially in terms of cost and maintenance compared to electronic-type counterpart. In addition, mechanical fuel system is better able to tolerate a low fuel quality which is potentially possible in emerging countries. A key enabler for the successful development of mechanical-type Tier3 diesel engine without any aftertreatment system was the advanced combustion design technologies driven by Doosan Infracore's own CAE (computer aided engineering) capabilities. The combustion system of the DX12 Tier3 engine features an optimized in-cylinder bowl geometry which enhances internal vertical motion to lead a better combustion quality, along with a proper targeting of fuel spray. This engine also employs a cooled EGR (exhaust gas recirculation) system with the Doosan Infracore's patented exhaust venturi technology that enables an adoption of a larger turbocharger to make the engine possible to bring a better breathing performance in high load conditions and to operate in a higher altitude without derating torque. Moreover, to ensure a further margin of engine-out NOx, the combustion system adopts a lower compression ratio by enlarging inner-bowl diameter. In this article, the design process and technical specifications applied in developing the combustion system of the DX12 Tier3 engine are described in detail.